The basic facts

On a rainy night in February, Trayvon Martin was returning to his father's girlfriend's house in Sanford, Florida, after a trip to the local 7-Eleven before the NBA all-star game. He carried a bag of Skittles and a can of Arizona iced tea for his little brother.

As he walked home, the unarmed 17-year-old spoke to his girlfriend in Miami via a hands-free earpiece on his cellphone he often used. He was spotted by George Zimmerman, a self-appointed neighbourhood watch volunteer, who was patrolling the gated community, the Retreat at Twin Lakes, in his vehicle.

Zimmerman called 911 to report what he described as Martin's "suspicious behaviour", telling the operator he looked like he was "up to no good, on drugs or something". Zimmerman was warned by the operator not to follow the teenager. But he did, getting out of his vehicle and taking a 9mm handgun with him.

Martin's girlfriend, who has provided her account to the family's lawyer Benjamin Crump, says Martin told her a stranger was following him. She told him to run home. Then the man reappeared behind him, she said. She then heard Martin say: "What are you following me for?" and someone else saying "What are you doing around here?". She then heard pushing, she said, because Martin's head set fell. At 7.16pm, four minutes after the call began, the phone went dead.

What happens in the last moment of Martin's life went unwitnessed by anyone other than Zimmerman.

According to neighbours, there was an altercation between the two men – Zimmerman, weighing 250lbs and armed, and Martin, weighing 140lbs and unarmed.

Witnesses and 911 calls record one of the men in great distress crying out for help.

One woman anxiously says she can hear someone calling for help while in the background, a wailing voice pleads, "No! No!"

By the time police arrive on the scene, at 7.17pm, Martin lay fatally wounded by a gunshot to the chest.

Zimmerman has admitted to the killing but told police he fired in self-defence.

Three and a half weeks later, Zimmerman is still free.

The police chief has said that, because Zimmerman has made a statement of self-defence, and there is no evidence to dispute that, he has not been arrested.

On Monday, following a national outcry over the case, the US justice department announced that it would investigate the case, along with the FBI.

What the tapes reveal

The 911 tapes, made public on Friday by police after Martin's family sued to have them released, include Zimmerman's initial call to emergency services, and provide a chilling account of what was in his mind as he followed Martin and, ultimately, shot and killed him.

Zimmerman, who called 911 from his truck, tells the dispatcher there's been a spate of burglaries and says: "There's a real suspicious guy. This guy looks like he's up to no good, on drugs or something. It's raining and he's just walking around looking about."

Zimmerman then tried to explain where he was.

Trayvon Martin. Photograph: AP

"Now he's coming towards me. He's got his hand in his waistband. And he's a black male … Something's wrong with him. Yup, he's coming to check me out. He's got something in his hands. I don't know what his deal is … These assholes, they always get away."

Zimmerman then said: "Shit, he's running," and the sound changes, suggesting he has left his vehicle to run after Martin.

Dispatcher: "He's running? Which way is he running?"

Zimmerman: "Down towards the other entrance to the neighbourhood."

Dispatcher: "Which entrance is that that he's heading towards?"

Zimmerman: "The back entrance … fucking [unintelligible]"

"Are you following him?" the dispatcher asked. Zimmerman replied: "Yep."

"OK, we don't need you to do that," the dispatcher warned.

In one of the 911 calls from neighbours, a woman tells operators: "There's someone screaming outside. I think they're yelling help, but I don't know. Just send someone, quick."

Anguished cries can be heard and then a single gunshot. She said: "There's gunshots. Just one". The operator says: "I can't hear any more yelling, can you?" and she responds "No."

The law

When asked why Zimmerman had not been arrested, the police said that he had made a statement of self-defence and that they have not established a case to dispute that.A 2005 Florida state law known as the "stand your ground" law permits anyone, anywhere, to use deadly force against another person if they believe their safety or life is in danger.

The law was signed by the then governor Jeb Bush, who called it a "good, common sense anti-crime" bill, but campaign ads against the law posted billboards warning Florida's tourists to avoid fights and "keep their hands in plain sight" if they end up in a disagreement. A further 16 states now have versions of this law.

Paul A Logli, president of the National District Attorneys Association, said of such laws: "They're basically giving citizens more rights to use deadly force than we give police officers, and with less review."

In that time, the stand-your-ground defence was used in 93 cases involving 65 deaths — and in most cases, it was used successfully.

In January, a former Broward County sheriff's deputy shot and wounded a homeless man inside a Häagen-Dazs ice cream shop in Miami Lakes. He said the man was threatening him and his family. Police said charges were unlikely in that case.

In 2008, a 15-year-old boy was killed during a shootout between two gangs in Tallahassee. Nobody was held accountable for the murder because a judge who cited the law dismissed the charges.

In one yet-to-be decided case, an unarmed man was shot and killed at a park near Tampa in a dispute over skateboarding rules in 2010. The victim's 10-year-old daughter watched her father die. A judge is currently considering whether the shooter merely stood his ground.

One problem with "stand your ground" laws is that they eliminate the concept of a "duty to retreat" from dangerous situations outside the home. Without a duty to retreat, a citizen has no obligation to stand down in the face of a threat. This means that prosecutors can no longer consider certain factors when determining self-defence, including the extent to which a person claiming self-defence may have aggravated the threat.

Prosecutors can have a hard time making a case if there is no one else around to contradict a person who claims self-defence, David Hill, a criminal defence attorney in Orlando told the Associated Press.

"If there is nobody around and you pull a gun, you just say: 'Hey, I reasonably believed I was under imminent attack. Hey, sorry. Too bad. But you can't prosecute me,'" Hill said.

Race

George Zimmerman says he acted in self-defence. Photograph: AP

Reports of the case suggest that Zimmerman was a vigilante with a false sense of authority and a fixation with young black males. Police records show Zimmerman had called 911 a total of 46 times between 1 January this year and the day he killed Martin, according to Mother Jones. Neighbours described him to the Miami Herald as being fixated on "young black men".

At least two websites have suggested that on an enhanced version the 911 tape, Zimmerman can be heard to say "fucking coons" as he chased after Martin, though the quality of the audio is too poor to confirm this and he could equally be saying "fucking phones" or something else.

Zimmerman's family have insisted that he is not a racist.

The case has also provoked claims of racism at the heart of the investigation. The officer in charge of the crime scene has been criticised before, in 2010, when he initially failed to arrest a lieutenant's son who was videotaped attacking a homeless black man.

Police officers are also alleged to have "corrected" a key witness when she said she heard the boy cry for help, according to ABC News. The station talked to the teacher and she confirmed that the officer corrected her and told her it was Zimmerman who cried for help, even though this has not been established as fact.

• Why did police tell Martin's father that they hadn't pressed charges because Zimmerman was a criminal justice student with a "squeaky clean" record? It wasn't strictly true. In 2005, Zimmerman was arrested for resisting arrest with violence and battery on a law enforcement officer, although those charges were dropped.

• Why did a police officer "correct" a witness who told him she had heard Martin scream for help and say it was Zimmerman she heard?

• What happened to Martin's cellphone? Reports abound that Martin's parents were not informed of his murder until they contacted police to report him missing, despite having his cellphone is feeding further scepticism about the police's conduct. It led New York Times columnist Charles Blow to tweet on Monday urging his followers to make his Tweet trend "Where is Trayvon Martin's cellphone?"